1. Field of the Invention
The present invention relates to a system for measuring the saturation of at least one gas, in particular the oxygen saturation, of blood present in a living body. The system may also be embodied for measuring the saturation of some other gas in blood--such as its carbon monoxide saturation, in addition to or possibly instead of measuring the oxygen saturation. The system is intended to enable noninvasive measurement. The term "noninvasive" means here that the measurement is carried out without using any instrument introduced into a blood vessel and accordingly solely with sensor means that are located entirely outside the living human or possibly animal body for which the measurement is carried out. In the planned use of the system for measuring a gas saturation, it will usually be the gas saturation of the arterial blood that is measured.
2. Description of the Prior Art
The hemoglobins present in the red blood corpuscles can bind various gases, such as oxygen and carbon monoxide--and transport them and give them up again. The blood may contain in particular oxyhemoglobin, in other words hemoglobin with bound oxygen, deoxyhemoglobin, in other words hemoglobin without oxygen, and carbon monoxide hemoglobin. The term "gas saturation" is understood as a value that provides a measure of the concentration or proportion of the hemoglobin that contains the applicable gas. For example, the oxygen saturation that is to be measured in particular may provide a measure of the ratio between the concentration of oxyhemoglobin and the concentration of the sum of the oxyhemoglobin and the deoxyhemoglobin, or of the total hemoglobin concentration. The degree of saturation that is equal to the value, shown in percentage, of the aforementioned ratio is often indicated as the measure of oxygen saturation.
For measuring the oxygen saturation or some other gas content in blood, an important factor is that light shone into a body and in particular into a blood vessel is scattered. The blood plasma and the red blood corpuscles contained in it, along with other blood cells, have various indexes of refraction, so that a beam of light upon entering a blood cell, for instance a red corpuscle, and on leaving it is normally deflected by refraction. The light can also be deflected by reflection and diffraction. The totality of all these deflection processes is called scattering. A beam of light shone into a blood vessel is typically scattered multiple times in the blood before it leaves the blood vessel again. If the light penetrates a red corpuscle, some of the light is absorbed.
In measuring the oxygen saturation using light, use is made of the fact that the hemoglobin containing bound oxygen, that is, the oxyhemoglobin, and the hemoglobin lacking oxygen, or in other words the deoxyhemoglobin, have different colors and correspondingly different light absorption spectra. If the absorption coefficients of the two types of hemoglobin are represented in the same diagram as curves as a function of the wavelength of light, the two curves intersect at so-called isobestic points, at a wavelength hereinafter called the isobestic wavelength which is approximately 805 nm.
Systems for noninvasive measurement of gas saturation in blood, in particular of its oxygen saturation, are known, for instance from the publication entitled "A New Noninvasive Back- Scattering Oximeter", by T. M. Donahoe and R. L. Longini, Proceedings of the Seventh Annual Conference of the IEEE/Engineering in Medicine and Biology Society, 1985, Chicago, Vol. 1, pp. 144-147, and U.S. Pat. No. 4,890,619 and British Patent A 2,228,314. These systems have sensor means, light transmission means in order to shine light into a body to be examined for at least one light transmission zone of the sensor means, and light reception means, in order to receive light scattered in the body in at least one light reception zone of the sensor means and measure its intensity using at least one photosemiconductor. The known systems also have electronic circuit means in order to ascertain the oxygen saturation, and possibly other variables as well, from the light intensities ascertained by the photosemiconductors.
The system known from the publication by Donahoe et al named above has two identical sensors. Each of them has two light-emitting diodes, which form light sources to generate light of two different wavelengths, and on the side of the sensor applied to the body being studied during measurement, they together define a light transmission zone through which the light generated can be shone into the body. When a measurement is carried out, one sensor, intended for measuring the oxygen saturation of the arterial blood, is heated with a heater container in it, while the other sensor is not heated and is intended to measure the oxygen saturation of the tissue.
The systems known from U.S. Pat. No. 4,890,619, already mentioned, also have two identically embodied sensors, each of which has one light receiver, embodied by a photodiode, and four or six light-emitting diodes disposed in a circle around it. The light-emitting diodes of each sensor form light sources to generate light at two different wavelengths, and together they define a circular light transmission zone in which light can be transmitted out of the sensor. The distance between the light reception zone, defined by the photodiode of the applicable sensor, and the annular light transmission zone is approximately equal to the mean radius of the light transmission zone. Some of the sensors also have ultrasound sources for heating the tissue with ultrasound. The systems known from U.S. Pat. No. 4,890,619 make it possible, depending on the arrangement of the two sensors, to measure the oxygen saturation with transmitted light passing through the body from one sensor to the other, or with backscattered light that is scattered from the body back into the same sensor in which it originated.
British Patent A 2,228,314, already mentioned, discloses a system intended, among other purposes, for measuring the oxygen saturation in the blood in the brain. The light transmission means of the system have a number of laser diodes, which are capable of generating light at various wavelengths for measuring the oxygen saturation. Each laser diode is connected to the inlet end of an optical fiber, whose outlet end is located on the surface of the head of a patient to be examined. The light reception means of the system have a number of optical fibers, with inlet ends located on the surface of the head of the patient. In the system primarily described, the optical fibers of the light reception means all lead from the head to the same single photodetector. It is also noted that a separate photodetector could be provided for each optical fiber of the light reception means, and that the computer that is part of the electronic circuitry of the system adds together the light intensities detected in measurement by the various photodetectors. The outlet ends of the optical fibers belonging to the light transmission means and the inlet ends of the optical fibers belonging to the light reception means are distributed more or less uniformly over the upper half of the head of the person to be examined. The system therefore has a number of light transmission zones and light reception zones; the light transmission zones and the light reception zones alternate along the circumference of the head, if seen in plan view vertically from above. There are also relatively large interstices between the closest-together light transmission zones and between the closest-together light reception zones. In operation of the systems, light is shone sequentially through the skull into the brain in the various light transmission zones. Light scattered there and transmitted back through the skull out of the head can then be intercepted by the various light reception zones. For example, if light is shone into the head at a light reception site located on the "equator" of the head, then at each light reception site, scattered light can be intercepted, which depending on the location of the applicable light reception site is backscattered, scattered to the side, or scattered forward.
In measurement with all the system that are known and are described above, light scattered not only by the red corpuscles but light that was scattered by dead skin cells, tissues, hair follicles, sweat glands, nerves and the like reaches the light receiver, or every light receiver. This latter light, not scattered by hemoglobins, provides no information on the oxygen saturation be measured, and it impairs the accuracy of measurement. For accurate determination of arterial oxygen saturation, the highest possible proportion of the light reaching the light receiver or receivers and evaluated to determine the oxygen saturation should therefore be scattered by the red corpuscles. As will be discussed in further detail hereinafter, the intensity of the light scattered by red corpuscles and reaching a light receiver through a light reception zone in measurement with backscattered light depends on the distance, measured along the surface of the body, of the light reception zone from the light transmission zone at which the light was shone into the body. As will also be discussed in detail hereinafter, the most favorable value for this distance depends on individual anatomical characteristics of a person being examined, on the site of the body selected for the measurement, and also on the instantaneous physiological condition of a person examined, which is subject to changes over time. The systems known from the various publications named above therefore have the disadvantages, for measurement with backscattered light, that the attainable accuracy of measurement can vary from one measurement to another, and that considerable measurement errors sometimes occur.
If the systems known from U.S. Pat. No. 4,890,619 are used to measure with transmitted light, they have not only more or less similar disadvantages to those of measurement with backscattered light, but above all the disadvantage that the measurements can be performed only on thin parts of the body, such as an earlobe or finger. In addition, measurements with transmitted light are relatively poorly suited for long-term continuous monitoring, because motion by the person being examined, for instance, often causes errors in measurement. The situation is similar for measurements with the systems of British Patent 2,228,314. Since when these systems are operated, the light has to pass twice through bony parts of the skull, the measurable light intensities are furthermore very slight, which additionally impairs the accuracy of measurement.
In the known systems, if the intensities of transmission of the various light-emitting or laser diodes, or the sensitivity of the photosemiconductors change over the course of time, this can also impair the accuracy of measurement. In particular, inaccuracies in measurement can arise if the transmission intensity at the various wavelengths varies differently, or if the spectral sensitivity of a photosemiconductor changes.
As mentioned, the sensors of the system described in the above-named publication by Donahoe et al is equipped with a heating coil. This makes it possible to warm the skin by conduction of heat and thereby increase the circulation. If the region of the skin containing the arteries and arterioles--that is, the dermis--is warmed by heat conduction from the surface of the skin, a temperature drop from the outside inward is created in the skin. If the heating is done solely by conduction of heat, it is therefore difficult and practically impossible to heat the region of the skin containing the arteries and arterioles to an optimal temperature for measuring the arterial oxygen saturation without causing harmful overheating in certain regions of the skin.
In the sensors known from U.S. Pat. No. 4,890,619, which are equipped with ultrasound sources, there is a rather large void between the ultrasound sources, located in the interior of a housing of the sensor, and the surface of the skin of the patient being examined. For measurement this void must either be filled with a gelatinous and more or less readily flowable filling composition of polyethylene glycol, or with water, which makes the measurement process more complicated, and if water is used requires a pump for introducing it. Since the ultrasound transmitted into the skin is not uniformly absorbed there, heating that occurs solely from transmitting ultrasound into the skin, like heating effected solely by heat conduction, may also produce a temperature distribution that, while different, is still uneven.
The supply of oxygen to tissues and/or organs of a living being depends not only on the oxygen saturation of the blood but also on the intensity of blood profusion, or in other words circulation. When the oxygen saturation is measured, it is therefore desirable to measure the intensity of blood profusion at the sam=time. With the known systems used for measuring oxygen saturation, however, the blood profusion cannot be measured, or if at all, then at best with only slight accuracy.
Similar problems to those in measuring oxygen saturation can also arise when some other gas saturation is measured, such as the carbon monoxide saturation of blood.